Conventionally, there has been known an air pollution control system for treating flue gas discharged from a boiler installed in a thermal power generation plant or the like. The air pollution control system includes NOx removal equipment that removes nitrogen oxides from flue gas discharged from a boiler, an air heater that recovers heat of flue gas having passed through the NOx removal equipment, a precipitator that reduces dust in the flue gas after heat recovery, and a desulfurizer that reduces sulfur oxides in the flue gas after dust reduction. As the desulfurizer, a wet desulfurizer that reduces sulfur oxides in flue gas by bringing an absorbent such as a limestone slurry into gas-liquid contact with flue gas has been generally used.
In flue gas discharged from the boiler, harmful substances such as mercury may be contained in a small amount other than nitrogen oxides and sulfur oxides. In the air pollution control system described above, therefore, as a method of removing mercury in flue gas, such a method has been used that a chlorinating agent is gas-atomized on an upstream of high-temperature NOx removal equipment in a flue gas duct, mercury is oxidized (chlorinated) on an NOx removal catalyst to prepare soluble mercury chloride, and mercury chloride is dissolved in an absorbent by a wet desulfurizer on a downstream (see, for example, Patent Literature 1).